Phylogeny and interfertility of North Atlantic populations of 'Ceramium strictum' (Ceramiales, Rhodophyta): how many species?

'Ceramium strictum' is an illegitimate name that has been used in a wide sense to encompass non-spiny, partially corticated North Atlantic populations of Ceramium. At the other extreme, several species have been recognized even within a single restricted geographic region (Skagerrak-Baltic). We combined molecular and interfertility analyses to assess relationships among samples of 'C. strictum' from five geographic regions (Skagerrak-Baltic, northeastern America, the Canary Islands, western Norway and the British Isles), with emphasis on the Skagerrak-Baltic Sea salinity gradient. Sequences of the nuclear ribosomal internal transcribed spacer 2 (ITS2) and the plastid Rubisco spacer region were obtained from these samples, from two other partially corticated species (C. siliquosum and C. deslongchampsii) and from two species described as fully corticated (C. pallidum and C. secundatum). In phylogenetic analyses using the most divergent species, C. deslongchampsii, as an outgroup, C. secundatum was basal to a clade containing all other taxa. The populations from the Skagerrak-Baltic region had virtually identical sequences and were interfertile, suggesting that they belong to the same species (C. tenuicorne). The populations of 'C. strictum' from the other geographic regions were highly divergent and may represent different species (tentatively named Ceramium strictum sensu Harvey, and Ceramium spp. 1 - 3). Formal taxonomic recognition of these putative species must await the results of thorough morphological analyses. Unexpectedly, C. tenuicorne was sister to C. siliquosum - C. pallidum in a well-supported clade that did not include the other 'C. strictum' populations. Thus, cortication is probably homoplastic and useless as an indicator of species relationships in this group of Ceramium.